The present invention relates to a method and an apparatus for delivering a liquefied compressed gas from a tube trailer or other supply source to a use point, such as a semiconductor fabrication tool or facility, and in particular to a method and an apparatus for detecting the occurrence of a liquid dry condition.
Reference is made to use of the invention for detection of an occurrence of a liquid dry condition in high-pressure tubes of hydrogen chloride (HCl). (The tubes are elongated cylinders which are stacked one upon the other on a trailer for transportation of chemicals and gases, as is well known in the industrial gas industry.) However, the invention can be used in connection with other types of liquefied compressed gases and other types of containers.
High-purity HCl is used for certain semiconductor processes, such as silicon epitaxial deposition. Bulk HCl is delivered to semiconductor customers in tube trailers, which include multiple tubes typically rated to 1800 psig. At 70.degree. F., HCl exists as a compressed liquefied gas under its own vapor pressure of 629 psig. Customers draw the vapor from each tube to feed their specific process applications, such that one tube serves as the source of gas until it is considered to be empty, when a crossover panel then changes (or crosses over) the source to the next available tube of gas.
It is desirable to determine when each HCl tube is near "empty" for several reasons. The customer desires to use as much HCl from each tube as possible, since they are billed per full trailer of product delivered, not by the amount of product that is used. It is undesirable, however, to draw product from tubes that are sufficiently empty that the product exists only in a gaseous phase, commonly referred to as a liquid dry condition. The liquid dry condition causes an increase in the levels of impurities of lower volatility in the gas stream, including an increase in moisture level, which causes corrosion. This could result in lower semiconductor yields.
A liquid dry point occurs when a pressurized liquefied compressed gas, such as HCl, in a container (such as a tube) is slowly vaporized for saturated gaseous supply, as follows. When substantial amounts of the HCl exist in the liquid phase, the pressure of the system remains relatively stable during the release and delivery of the saturated gaseous HCl because the liquid portion vaporizes with the input of heat from the environment. Eventually a physical state occurs where all of the liquid has been vaporized and the remaining HCl exists in an entirely unsaturated, gaseous phase. At precisely this moment, a liquid dry condition has occurred, wherein the pressure of the container decays rapidly thereafter.
Moisture and volatile metallic compounds can typically increase significantly after the liquid dry point is reached in liquefied compressed gas such as HCl. When a two phase system exists in a container (such as tube trailer), a vapor liquid equilibrium is maintained. Contaminants such as moisture and volatile metals have very low partition coefficient and concentrate more in the liquid phase leaving the vapor phase much cleaner. (Partition coefficient is the ratio of the concentration of a volatile component in the gaseous phase to its concentration in the liquid phase when the system is in vapor liquid equilibrium.) These contaminants get more concentrated as HCl preferentially vaporizes as ultra-pure product during transfer and delivery to the use point. Upon reaching the liquid dry point, due to the absence of the liquid phase, moisture and volatile metals are free to pass with the delivered gas. Therefore, when liquid dry point is reached, higher than normal levels of moisture or any volatile metals are experienced in the delivery of final gaseous product from the source of supply (such as tube trailers). It is therefore desirable to detect the approach or obtaining the liquid dry condition.
Devices such as mass flow meters or mass flow totalizers have been unreliable in HCl service and therefore cannot be used to detect liquid dry conditions based on mass balance calculations. Typically, a weigh scale is used to identify when a tube is approaching a liquid dry condition. However, this us ally requires leaving a nominal liquid heel in the tube, making it less than an optimal solution. Also, the purchase and installation of a scale requires a large capital investment.
An alternate method of identifying when a tube is approaching a liquid dry condition is disclosed in U.S. Pat. No. 5,359,787 (Mostowy, et al.). Sensors, such as thermocouples and pressure transducers, are provided for the source supply (e.g., a tube trailer) and the ambient temperature conditions. The ambient temperature at the source supply is sensed, the temperature of the chemical from the source supply is sensed, and the relative change in pressure over selected time intervals is sensed and transmitted to a digital computational controller. These values are compared against preset values for ambient temperature, source of supply temperature and pressure indicative of the liquid dry point (gas phase), and when the sensed values exceed the prescribed preset values which indicate the liquid dry point is reached, the controller provides an appropriate alarm signal.
U.S. Pat. No. 5,359,787 teaches that the liquid dry point also can be calculated by determining and inputting to the digital computational controller the volume of the tube trailer, the weight of the trailer during transfer of the liquefied compressed gas, and the ambient temperature at the tube trailer, as well as the temperature of the chemical leaving the tube trailer and entering the delivery conduit. These values are compared to preset values already input into the controller which represent approach to substantially gas phase of the chemical (meaning that at least some small amount of chemical is still in the liquid phase). When the sensed values meet or exceed the preset values so as to indicate the approach to the liquid dry point (substantially a gas phase), the system generates an alarm signal.
Neither of the methods disclosed in U.S. Pat. No. 5,359,787 for identifying or determining the liquid dry point works as well as the present invention, which provides a more exact method for detecting the occurrence of a liquid dry condition using a more quantitative approach.
It is desired to have a more cost effective, reliable method of detecting the occurrence of a liquid dry condition in a container of liquefied compressed gas, such as HCl.
It is further desired to have a more cost effective, reliable method of delivering a high-purity industrial chemical in gaseous phase from a transport vehicle having multiple tubes which contain the chemical in a liquefied compressed gas phase. It also is desired to have an improved transport vehicle for delivering a high-purity industrial chemical in gas phase.
It is still further desired to direct the changing (or crossover) of tubes in a tube trailer or other bulk delivery system in an optimal manner.